A conventional solar thermal electric power generation system 101 shown in FIG. 15 has a configuration wherein sunlight is collected by means of a concentrating type of heat collecting unit (hereinafter referred to as simply “heat collecting unit”) 102, a heating medium absorbs collected sunlight as thermal energy, and the heating medium is supplied to a heat exchanging device 103 to generate steam by utilizing heat of the heating medium. Saturated steam generated by the heat exchanging device 103 is then superheated by means of a superheater 104. A steam turbine 105 is driven by such superheated steam to generate electric power. In the figure, reference characters 106 and 107 denote an electricity generator and a condenser, respectively.
Methods of collecting incidental solar radiation are roughly classified into a central receiver type and a parabolic trough type. The heat collecting unit 102 of the parabolic trough type uses trough-shaped reflectors 102a having a parabolic section in an X-Y plane and configured to reflect sunlight thereon to collect them on its focal point. Heat absorbing tubes 108 each extending through the focal points of reflectors 102a along the Z-axis allow a heating medium to pass therethrough to collect solar heat. The heat absorbing tubes 108 and heating medium supply piping 109 connected thereto allow the heating medium to circulate between a heat exchanging device and the heat collecting unit. A special synthetic oil is generally used as the heating medium. For example, The heating medium absorbs solar heat to reach a high-temperature condition of about 400° C., releases heat to generate steam in the heat exchanging device 103 thereby to assume a low-temperature condition of about 300° C., and returns to the heat collecting unit 102.
As can be seen from FIG. 16, a plot of the solar energy density variance during one day, the conventional solar thermal electric power generation system can operate only during day time from sunrise to sunset. For this reason, the system is stopped at night and restarted the next morning. FIG. 16 plots the solar energy density variance during one day at a region in North Africa. Curves plotting mean energy densities in July and December are shown in FIG. 16, and curves plotting mean energy density variance in other months are considered to fall within the range between the two curves shown.
As shown, the intensity of solar thermal energy reaching the heat collecting unit 102 varies from zero to maximum during one day. Therefore, the electric power generation system 101 is usually designed to have such a capacity as to generate electric power at a mean solar energy intensity level. As is often the case, the system 101 is designed to store surplus energy in excess of a mean solar energy intensity level as thermal energy in a large-scale and expensive heat storage system 110 and release heat thus stored to generate steam as sunset approaches, thereby making it possible to continue electric power generation. However, in actuality limitations on the system investment cost and running cost limit the heat storage capacity to about 4 to 6 hours in terms of electric power generating duration and, therefore, electric power generation cannot continue day and night.
In an attempt to solve this problem, for example, European Patent Laid-Open Publications Nos. 0750730 and 0526816 have proposed integrated solar combined cycle electric power generation systems in each of which the above-described steam turbine electric power generation relying upon solar heat is combined with the gas turbine combined cycle electric power generation. Such a new concept of a solar thermal electric power generation system is intended to generate electric power even during nighttime or cloudy days, during which solar heat cannot be utilized, by a combination of the gas turbine electric power generation with the steam turbine electric power generation by utilizing steam generated in a waste heat recovery boiler. The system thus configured can be expected to continue electric power generation day and night. Also, the integrated system can reduce the fuel consumption of the gas turbine; hence, the carbon-dioxide emission amount can be reduced by the maximum use of solar heat.
However, such an integrated solar combined cycle electric power generation system includes a heat collecting unit configured to generate saturated steam directly from water and supply it to a steam turbine without using a special heating medium or a heat exchanging device. This kind of electric power generation system according to the aforenoted European Patent Laid-Open Publication No. 0750730 is configured to mix saturated steam with steam generated from a high-pressure turbine for superheating the saturated steam before supplying it to the steam turbine. On the other hand, another kind of electric power generation system according to the aforenoted European Patent Laid-Open Publication No. 0526816 is configured to mix saturated steam with steam generated from a high-pressure turbine and then superheat the saturated steam by means of a reheater of the waste heat recovery boiler before supplying it to the steam turbine.
Irrespective of the conventional solar thermal electric power generation system and the integrated solar combined cycle electric power generation system, there exists an unavoidable problem that relates to the condition of solar radiation onto the surface of the Earth incidentally changing (with time) during daytime. In the solar heat collecting unit, heat transfer from solar heat to steam or other heating medium is mostly based on solar radiation condition. Accordingly, the temperature of steam or other heating medium absorbing solar heat fluctuates in exact response to changes in the condition of sunshine onto the surface of the Earth as a natural phenomenon. Since such a fluctuation occurs according to nature, it is difficult to accurately predict the time at which the fluctuation occurs and the degree of the fluctuation. As a result, the condition of steam (including temperature, pressure, wetness, dryness and the like) to be supplied to the steam turbine fluctuates, which causes the generated electric power to fluctuate. If a vigorous fluctuation occurs in the condition of steam, the waste heat recovery boiler or the steam turbine may be damaged thereby.
With the two systems disclosed in the aforenoted Laid-Open Publications Nos. 0750730 and 0526816, for example, the condition of steam (including temperature, pressure, dryness and the like) generated in a heat absorbing tube associated with the heat collecting unit fluctuates and steam loses its heat while being fed from the heat collecting unit to the steam turbine. As a result, the system according to the Laid-Open Publication No. 0750730 causes the condition of steam to fluctuate after mixing with the steam generated from the high-pressure turbine. The system according to the Laid-Open Publication No. 0526816 causes the condition of steam to fluctuate at the inlet side of the reheater thereby giving influence on the waste heat recovery boiler. That is, when the sunshine condition fluctuates largely or frequently, the condition of steam generated in the heat collecting unit fluctuates likewise, which makes it difficult for the whole of the solar thermal combined cycle electric power generation system to serve continuously for safe operation.
Such a fluctuation in sunshine conditions, which is caused by clouds, a sandstorm or a like factor, causes the amount of collected heat to decrease steeply. Further, when each of the aforementioned reflectors is bent by wind incidentally, sunlight cannot be sufficiently concentrated on the aforementioned heat absorbing tube. This also causes a fall of the temperature of the heating medium or the like. Since such a temperature fall possibly occurs at short intervals, a required amount of heat cannot be timely taken out of the aforementioned heat storage system in a short time, hence, the stored heat cannot be utilized to effectively suppress the temperature fluctuations of the heating medium.